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Current state: Under Discussion Accepted [VOTE]: 167 Add interface for the state store restoration process

Discussion thread:here

JIRA:

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public interface StateRestoreListener {

    void onRestoreStart(TopicPartition topicPartition, StateStore storeName, long startingOffset, long endOffset);

    void onBatchRestored(TopicPartition topicPartition, String storeName, long batchEndOffset, long numRestored);

    void onRestoreEnd(TopicPartition topicPartition, String storeName, long totalRestored);
}

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The changes also include adding a setter method on the KafkaStreams object:, named setGlobalStateRestoreListener to reinforce the fact the listener is for the entire application

public void setStateRestoreListenersetGlobalStateRestoreListener(final StateRestoreListener stateRestoreListener)

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For single action state restoration, there is AbstractNotifyingRestoreCallback

public abstract  class AbstractNotifyingRestoreCallback  implements StateRestoreCallback, StateRestoreListener {


    @Override
    public void onRestoreStart(TopicPartition topicPartition, String storeName, long startingOffset, long endingOffset) {

    }

    @Override
    public void onBatchRestored(TopicPartition topicPartition, String storeName, long batchEndOffset, long numRestored) {

    }

    @Override
    public void onRestoreEnd(TopicPartition topicPartition, String storeName, long totalRestored) {

    }
}

For the corresponding bulk action state restoration, we have AbstractBatchingRestoreCallback

public abstract class AbstractBatchingRestoreCallback implements BatchingStateRestoreCallback, StateRestoreListener {

    @Override
    public void restore(byte[] key, byte[] value) {
        throw new UnsupportedOperationException("Single restore not supported");
    }

    @Override
    public void onRestoreStart(TopicPartition topicPartition, String storeName, long startingOffset, long endingOffset) {

    }

    @Override
    public void onBatchRestored(TopicPartition topicPartition, String storeName, long batchEndOffset, long numRestored) {

    }

    @Override
    public void onRestoreEnd(TopicPartition topicPartition, String storeName, long totalRestored) {

    }
}

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The first use case is user updates of the restore progress - In this case users of a Kafka Streams program application want to get notification receive updates of the progress, reporting status back to restoration progress and publish those updates to a UI for example.  The StateRestoreListener set via the KafkaStreams.setStateRestoreListener method will be a setGlobalStateRestoreListener method functions as a single, global listener .  This means for reporting purposes there will only be one listener reporting on the restoration status for all state stores in an application.  Additionally, the StateRestoreListener also reports on the bootstrapping progress of any GlobalKTables defined in the application.   

 The second use case is internal state store management, closing and re-opening a RocksDB instance for bulk loading with different configuration settings for example.  In this case implementors of a custom store want notification of restoration start, progress and ending for state manage purposes.  In this case, the StateRestoreListener implementation is  used used internally by the given state store.  In this use case, users can specify a StateStoreListener per store, but the intent here is not for reporting but for internal state management.    

To use the listener functionality users will implement the StateRestoreListener interface in addition to the StateRestoreCallback or BatchingStateRestoreCallback interfaces when constructing their callbacks.  Providing the callback is still done via the ProcessorContext.register method.  

During the restoration process the type of the restoreCallback is inspected and if it implements the StateRestoreListener then the listener methods are executed.  With this in mind, the StateStoreListener API can be called in two places (although two different implementations); 

1. If the instance level listener is set via the KSteam.setStateRestoreListener method, then that listener will be executed for each poll call.
2. If the provided state-store-level callback extends the StateRestoreListener interface, then those listener methods triggered for each poll call that is restoring that specific store as well.

Compatibility, Deprecation, and Migration Plan

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